1. Field of the Invention
This invention relates generally to the fabrication of micro electromagnetic devices such as micro electromagnetic coils, which can be used as transducers within micro electromechanical systems (MEMS).
2. Description of the Related Art
The majority of MEMS transducers have been electrostatically driven due to their ease of fabrication and integration with other micro components. On the other hand, the fabrication of magnetically actuated transducers has generally been avoided due to lack of processing knowledge and the difficulty of integrating magnetic components with other microsystems and circuits. In spite of poor scaling of magnetic forces, there is at present a growing interest in magnetic micro-devices, mainly due to their inherent unique advantages, among which is the ability of magnetic-based actuators to generate large magnetic forces.
Miniaturization needs to take into consideration many factors other than down-scaling, particularly when magnetic forces are utilized in applications other than microactuators, such as bio-medical applications. As Barbic points out (Mladen Barbie, “Magnetic wires in MEMS and bio-medical applications,” J. of Magnetism and Magnetic Materials, 249 (2002) 357-367) magnetic components can generally create larger forces at larger distances than electrostatic-based components, because the energy stored in a gap between magnetized components is larger than that stored between charged plates, which is true for a gap size as small as 10 nm.
In addition, since the magnetic materials are responsive to magnetic fields and magnetic field gradients generated by current carrying wires, magnetic devices tend to be low impedance current-driven devices, rather than being high impedance voltage controlled devices, which is the case for electrostatic actuators.
Several attractive features of micro-magnetic devices that make them strong candidates for use in biomedical applications are:
1. The small size of micro-magnetic wires generates large magnetic field gradients and correspondingly large forces, since Fmag∝∇B.
2. Magnetic devices can generate large forces at larger distances than electrostatic devices, which only exert large forces at small distances.
3. Using magnetic devices in bio-medical separation techniques is more efficient, non-destructive and gentle in nature and exhibits excellent selectivity. The magnetic force is exerted only on the target (e.g. a magnetic bead-cell complex) since this is the only magnetic object in the analyzed sample that exhibits a μ different from the surrounding environment. In the case of electrostatic devices, the electrostatic forces are exerted (unequally) on both the target and other objects in the sample, leading to unavoidable attraction of undesired particles together with the target, thus resulting in poor selectivity.
Magnetic micro-actuators and sensors have been fabricated for many years and, until now, they were realized simply as scaled-down versions of their macroscopic counterparts, such as electromagnetic motors, magnetometers and sensors. Their electromagnetic interaction is mainly due to the use of external micro-coils or micro-permanent magnets. It is well known, however, that it is a challenging, time consuming and expensive task to construct micro-coils using classical methods, or to hand-machine large pieces of permanent magnetic material into micro-permanent magnets, for the purposes of constructing micro-devices. The fabrication of micro-coils, in particular, poses critical problems, because the coils are three-dimensional and micro-fabrication is typically two-dimensional. The invention proposed herein not only eliminates these problems, but also integrates ferromagnetic materials into the MEMS devices. In doing so, the invention benefits from the advantages of silicon micro-machining technologies well known in the integrated circuit (IC) industry.
In order to make use of and benefit from silicon (Si) based IC technology in the fabrication of micro-magnetic devices, problems relating to usable materials must be solved. In particular, consideration must be given to the Joule heating that occurs when injecting large currents into highly resistive structures and to the compatibility of bulk magnetic materials with micro-scale processing. In an initial approach to the invention, two components are obtained: 1) highly conductive materials, such as Cu, which are needed to withstand the large currents required to produce significant magnetic fluxes, are employed in forming a planar micro-coil that is embedded in Si-micromachined structures; and 2) a magnetic core is realized using soft (low coercivity) magnetic materials such as CoNiP alloys, which must also be compatible with semiconductor fabrication techniques.
The fundamental components of any electromagnetic device are the coil and the core. Coils for generating magnetic fields are of two types, planar and three-dimensional. Planar coils are generally formed in either a spiral shape (square or circular coil of decreasing dimension) or meander shape (convoluted windings generally in the form of repeated loops). In the case of planar micro-coils, standard IC technology can be used and a current can be injected to produce an appropriate magnetic flux. In the case of three-dimensional micro-coils, however, a more advanced fabrication process is required, employing technologies such as wafer bonding and through-via connections. Since all coils, planar or three-dimensional, macroscopic or microscopic, are current-driven devices and since the maximum injected current is limited by Joule heating, the coil resistance must be minimized. To do this, it is desirable to construct coils of large cross-sectional area, short length and to use the highest conductivity material available within the fabrication process.
Most of the planar micro-coils have been fabricated by depositing the conductor material (e.g. a metal) as a thick film on top of a Si surface. These planar micro-coils can be formed with or without a core. For example, spiral inductors have been fabricated for use in NMR microscopy. L. Renaud et al., “Implantable planar rf microcoils for NMR microspectroscopy,” Sensors and Actuators A 99 (2002) 244-248, describe the use of micro-coils in the analysis of small volume biological samples. The micro-coil is formed by depositing a Cu seed layer on a Ti adhesion layer, on an oxidized Si substrate. A thick layer of photoresist is then formed on the Ti/Cu layers and patterned to form a spiral mold. Cu is then electroplated into the mold and the photoresist and Cu seed layer are subsequently removed to insulate the wires. Using a substantially similar method, C. R. Neagu et al., “Characterization of a planar microcoil for implantable microsystems,” Sensors and Actuators A 62 (1997) 599-611, describe inductive coupling between an external transmitting coil and an implanted receiving planar micro-coil so as to transmit energy to the material containing the implanted coil
Neagu's method for forming a micro-coil has many disadvantages, including:
(1) The thickness of the conductor is determined by the photoresist.
(2) The final micro-coil has a non-planar upper surface, which can adversely affect subsequent processing steps and integration of additional components.
C. C. Sheng et al. (“Electromagnetic optical switch for optical network communication,” J. Magnet. Mater., 239, 2002, 610-613) design and fabricate an electromagnetic optical switch with NiFe (permalloy) on a vibrating suspension diaphragm and a planar micro-coil generating the magnetic force to produce the diaphragm displacement. The system was realized using many complicated fabrication processes, including electroplating, bulk micromachining, excimer laser ablation and wafer bonding. The coil does not have a magnetic core, which results in significant flux leakage.
D. J. Sadler et al. (Micromachined Semi-Encapsulated Spiral Inductors For Micro Electro Mechanical Systems (MEMS) Applications, IEEE Trans. on Magnetics, Vo. 33, No. 5, September 1997, 3319-3321) design and fabricate a semi-encapsulated spiral inductor, formed on pyrex glass that is bonded to a Si wafer. The multi-step process includes the following: (a) seed layer deposition on a substrate; (b) photolithography to form an electroplating mold using a first mask; (c) copper coil electroplating; (d) resist and seed layer removal; (e) via patterns formation using second photolithography process; (f) high temperature curing; (g) seed layer evaporation; (h) top magnetic and via patterning using third patterning process. As can be seen, this process requires several stages of photoresist and photolithography, which places stringent requirements on such factors as surface planarization.
Photoresist is also used as a patterning, insulating and planarizing material in the fabrication of planar micro-coils. Filas et al. (U.S. Pat. No. 6,495,019) teaches a process that allows the formation of a variety of integrated CMOS and micromagnetic components, particularly planar coils and transformers. The process involves forming a first patterned photoresist layer within which to electroplate a magnetic layer onto a seed layer. Next, a second patterned photoresist layer is formed as an insulating/planarizing/support layer over the magnetic layer. Finally, a conducting coil is formed by electroplating into the patterned second photoresist layer. In some specific applications, the resulting planar micro-coil was sandwiched between two layers of magnetic material, which shield the coil and restrict the magnetic field produced by the coil to the region between the layers.
The design of these types of micro-coils suffer from several limitations, the most severe being the photolithographic processes required to form high aspect-ratio electroplating molds from the photoresist layers. The photoresist insulation layer between the micro-coils must fulfill many requirements, including photostructureability, ability to support vias for interconnections between different micro-coil layers, the ability to supply sufficiently planarizable surfaces so that successive layers can be formed and precisely photolithographed and good dielectric properties (e.g. low dielectric constant and high breakdown voltage.) Finally, the micro-coils formed by the method have a size on the order of millimeters, which limits their ability to produce the desired magnetic field gradients.
Recent research has also focused on the fabrication of 3-dimensional micro-coils. For example, Chong H. Ahn et al. (“A New Toroidal-Meander Type Integrated Inductor With A Multilevel Meander Magnetic Core,” IEEE Transactions on Magnetics, Vol. 30, No. 1, January 1994) forms a toroidal inductor, with a multi-level meander magnetic core (a multilevel core linking planar coils, rather coils linking a planar core) was formed on a silicon wafer using multi-level metal interconnect schemes. A complex 6-step fabrication process involves (a) polymide deposition on a Si wafer; (b) plasma etching a plating mold through the polymide using an Al mask; (c) plating a lower magnetic core into the mold; (d) deposition and patterning of a conductor about the core; (e) plating vias; (f) plating an upper core onto the lower core to form the meander. The inductor size was 4 mm×1 mm, with only 30 turns and the magnetic core material was electroplated Ni81Fe19, producing an inductance of 30 nH/mm2 at 5 MHz.
Another 3-dimensional air-core micro-coil has been fabricated by using the deformation of a sacrificial thick polymer and electrodeposition. Nimit Chomnawang and Jeong-Bong Lee (“On-chip 3D air core micro-inductor for high-frequency applications using deformation of sacrificial polymer,” Proc. SPIE, Vol. 4334, 2001, 54-62) describe the formation of an inductor coil by first forming patterned, electroplated planar bottom conductor pieces, then connecting the bottom conductor pieces with an upper curved conductor formed by electroplating conductor pieces on a supporting convex surface. The bell-shaped convex surface is formed by curing a positive photoresist originally deposited in a rectangular mesa formation. The deformed photoresist support core is then stripped away, leaving a completed coil with an air core. The large number of vias required to interconnect all sections of the coil dramatically increases the coil resistance, limits the magnitude of the current that can be injected and, therefore, limits the maximum magnetic flux density.
M. Babric, J. J. Mock, A. P. Gray and S. Schultz (“Scanning probe electromagnetic tweezers,” Appl. Phys. Lett., 79 (12), 2001, 1897-1899) describe the fabrication of a solenoidal 3D micromagnetic manipulator wherein a 25 micron copper wire is wound around a 50 micron diameter soft-magnetic wire. The resulting solenoid was used to manipulate micro-magnetic particles. The performance of such a fabrication is limited both by the number of turns that can be wound around the magnetic core and the feasibility of integrating the fabrication to other microcomponents.
More recently, Nuytkens et al. (U.S. Pat. No. 6,696,910) fabricated a toroidal-shape inductor on a printed circuit board (PCB) composed of two dielectric layers sandwiching a ferromagnetic layer. A toroidally wound coil is formed by passing conducting leads through both dielectric layers by means of vias, so that the ferromagnetic layer becomes the core of the coil. Primary and secondary coils can also be formed in this way. The nature of the PCB may not permit the device to operate at high current densities. An additional problem arises from the fact that the ferromagnetic material forming the core is nickel that is deposited by electroplating or electroless plating. The use of various chemical agents in the nickel plating process introduces phosphorus in the final layer which can adversely affect its magnetic saturation (see, e.g. Wolfgang Riedel, “Electroless Nickel Plating,” ASM International, Finishing Publications Ltd., England, 1989, pp. 104-106.
As can be seen from the above citations, there are several limitations and disadvantages to be found in prior art structures and methods which can be summarized as follows:
(1) The dimensions of the fabrications produce limited magnetic field gradients and, in consequence, limited magnetic forces.
(2) The designs are basically scaled down versions of macroscopic designs and, as such, require complicated fabrication processes.
(3) The problems associated with the complicated designs and process steps also result in uneven surfaces, which is a disadvantage when subsequent process steps are required.
(4) None of the prior art methods are able to efficiently generate high magnetic field gradients along with planarized surfaces compatible with the integration of additional circuitry and components on the same chip.